1. Field of the Invention
This invention relates to a method of making intrafiber crosslinked cellulose and the product resulting from the process. The invention is especially directed to a crosslinked cellulose having a high wet and dry resilience, high bulk, and superior absorbency.
2. General Discussion of the Background
It is known in the art that resilient bulking fibers are useful for the preparation of bulky and more absorbent paper structures. Such paper structures are useful for the manufacture of products such as handsheets, towels, tissues, filters, paperboard, diapers, sanitary napkins, hospital dressings and the like. Crosslinked cellulose materials may be generated by reacting cellulose fibers with crosslinking agents which are capable of combining with at least two hydroxyl groups within a single cellulose molecule, or between adjacent cellulose molecules. The crosslinking agent must be difunctional such that it will react with at least two of the hydroxyl groups in the cellulose molecule to form the crosslink.
One method for obtaining resilient bulking fibers is by crosslinking cellulose fibers by treatment with a chemical compound. U.S. Pat. No. 3,819,470 discloses cellulosic fibers having a substantive polymeric compound reacted with and attached to the fibers. The modified fibers are characterized by reduced swellability and a reduced capability of natural fiber-to-fiber bonding when compared to unmodified cellulosic fibers. U.S. Pat. No. 4,431,481 discloses modified cellulosic fibers produced by treating the fibers with copolymers of maleamic acid. Other known techniques include treatment of fibers with cationic urea formaldehyde resins, (U.S. Pat. No. 3,756,913), methylol ureas and melamines (U.S. Pat. No. 3,440,135), formaldehyde (U.S. Pat. No. 3,224,926), with the condensation product of acrolein and formaldehyde, (U.S. Pat. No. 3,183,054), bis-acrylamides (Eur. Patent No. 213,415), and treatment with glyoxal or glutaric dialdehyde (WO 88104704, U.S. Pat. No. 4,822,453, and U.S. Pat. No. 4,853,086). Copending U.S. patent application Ser. No. 07/607,268 discloses a crosslinking process in which the crosslinking agent is dimethyldihydroxy-ethylene urea (DMDEU).
A drawback of many of these prior crosslinking agents is that they are inefficient crosslinkers or are toxic. The problem of toxicity is a particular concern with formaldehyde crosslinkers. Formaldehyde is toxic when inhaled, and can be strongly irritating to the skin and mucus membranes. Concerns have also been expressed that formaldehyde is teratogenic and carcinogenic. Public anxieties about environmental safety and occupational hazards have provided a special impetus to find new, non-formaldehyde crosslinkers.
Three techniques have generally been used to produce intrafiber crosslinked material. They are dry crosslinking, aqueous crosslinking, and crosslinking in a non-aqueous solution. In the dry crosslinking process, the cellulose fibers are crosslinked while in an unswollen, collapsed state. Dry crosslinked fibers are stiffened by crosslink bonds, such that absorbent structures made from the fibers have high wet and dry resilience, and low fluid retention. Aqueous solution crosslinked fibers are produced by crosslinking fibers in an aqueous solution, such that the swelling effect of water causing the fibers to be crosslinked in a swollen condition. Compared to dry crosslinked fibers, aqueous crosslinked fibers have increased flexibility, reduced stiffness, higher fluid retention, and lower wet and dry resilience. Nonaqueous crosslinking occurs when individualized, dehydrated, nonswollen fibers are contacted with a crosslinking agent in a substantially nonaqueous solution. The resulting fibers are stiff and exhibit high wet and dry resilience.
An example of using dry crosslinking technology is U.S. Pat. No. 3,440,135 to Chung. This patent discloses a technique of pre-soaking cellulose fibers in an aqueous solution of a crosslinking agent to reduce interfiber bonding. The treated fibers are then aged prior to carrying out a drying stage, in which the fibers are heated to effect crosslinking. The Chung patent suffers from the drawback that the wet fiber mat must be stored between 16 and 48 hours, in order to minimize nit formation resulting from incomplete difiberization.
Another example of dry crosslinking technology is U.S. Pat. No. 3,224,926 to Bernardin. That patent describes treating cellulosic material with a crosslinking agent such as formaldehyde or dimethylolurea. Individualized, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with a crosslinking agent, dewatering and then mechanically defiberizing the fibers, and then drying the fibers at an elevated temperature to crosslink the fibers while they are substantially individualized. The fibers are crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. The products made by this dry crosslinking process exhibit high absorbency and high wet and dry resilience.
An example of an aqueous crosslinking process is U.S. Pat. No. 3,241,533 to Steiger, in which the cellulose fibers are crosslinked in an aqueous solution with a crosslinking agent and a catalyst. The product made from this process was said to exhibit high fluid retention and great flexibility compared to a product made from a dry crosslinking process. Finally, an example of a nonaqueous crosslinking process is U.S. Pat. No. 4,035,147 to Sangenis et al. In this process, the lack of water present in the solution keeps the cellulose fibers in a state similar to that in the dry crosslinking process. While in the nonaqueous solution, the cellulose fibers are crosslinked with a crosslinking agent and a catalyst. Like dry crosslinked fibers, the nonaqueous crosslinked fibers are very stiffened by crosslink bonds, and absorbent materials made from these fibers have high wet and dry resilience.
Various devices are known in the art for treating fibers with crosslinking agents in mat form and thereafter breaking the mats into individual fibers. For example, U.S. Pat. No. 3,440,135 to Chung discloses a mechanism for applying a crosslinking agent to a cellulosic fiber mat. The mat is then aged and passed (while still wet) through a fiberizer, such as a hammermill to fiberize the mat. The resulting loose fibers are then dried in a two stage dryer. The first dryer stage is at a temperature sufficient to flash water vapor from the fibers and the second dryer stage is at a temperature that cures the crosslinking agent. A cyclone separator then separates the fibers from the gas for subsequent collection. The Chung apparatus suffers from the drawback of requiring the inconvenient and costly storage of wet fiber mats (e.g. in roll form) for a substantial period of time in order to minimize nit formation.
Unfortunately, fiberization processes known in the art which employ currently available fiberizing or comminution machinery yield crosslinked fibers that have too many nits and knots to be acceptable for many uses. A probable reason is that such machinery has excess dead space where fibers are excessively pressed together and/or has localized regions of elevated temperature hot enough to cause premature curing of the crosslinking agent while fibers are in intimate contact with each other. Since fiberization is performed on a mat that is still wet with the uncured crosslinking agent, dead spaces and hot spots in the fiberizer would encourage the formation of interfiber bonds, which form nits, that virtually cannot be broken by downstream equipment.
Interfiber bonding in a conventional fiberizer apparatus can also lead to production of excessive amounts of "fines," which are undesirably short fibers due principally to fiber breakage. Crosslinking imparts substantial brittleness to cellulose fibers, which thereby exhibit limited compliance to mechanical stresses. Nits are especially susceptible to mechanical stresses because of their density which is much greater than the density of individual fibers. Excess fiber breakage and fines not only degrade absorbency but can substantially reduce the loft and resiliency of a product made from crosslinked fibers.
Hence there is a need for a process of producing a product made of individualized crosslinked cellulose fibers that have minimal nits and knots. It is therefore an object of the invention to produce treated fibers, such as intrafiber crosslinked cellulose, having a nit level lower than levels obtainable with existing equipment. There is also a need for such an apparatus that will produce fibers from a mat comprised of crosslinked cellulose while not causing significant breakage of individual fibers of the mat.
It is yet another object to provide crosslinking agents that are less toxic and provide a product having high wet and dry resilience, high bulk, and superior absorbance.
Finally, it is an object to provide a crosslinking process that operates at a pH that is compatible with standard unmodified papermaking equipment.
These and other objects of the invention will be understood more clearly by reference to the following detailed description and drawings.